Despite this possible role of increased dead space ventilation, the major impact on ventilatory efficiency in cyanotic patients is most likely due to alveolar hyperventilation. Considering the correlation of end-tidal partial pressures for oxygen and carbon dioxide with ventilatory efficiency under exercise among patients and control subjects (Fig 4), alveolar hyperventilation with resulting alveolar hy-pocapnia can be expected as the most important mechanism influencing ventilatory efficiency in our patients. comments

The shunting of oxygen-poor and carbon dioxide-rich blood necessitates an adequate hyperventilation of the pulmonary venous blood. Alveolar hyperventilation, as reflected in decreased PetC02 and increased PeT02, provides a normalization of the PaC02 in the systemic circulation, which obviously is the major control mechanism in these patients and is not overcome by hypoxic ventilatory drive.
In our study, the extent of hypoxemia did not reflect the symptomatic state completely, while peak V02 did correlate with it to some extent. Overall, the summation of disease-related factors mentioned above seems to play a major role in determining the range of symptoms in these patients. Despite the complexity of these alterations, ventilatory efficiency at rest and during exercise can reliably be used to quantify functional impairment in these patients.
Other investigators have described resting hy-pocapnia in comparable patients. The small, albeit significant, decrease in resting HCO3~ suggests a mild respiratory-compensated metabolic acidosis. Despite a pH within the normal range, a significant correlation between resting pH and resting VE, respectively, resting ventilatory efficiency, was present.
pH-sensitive brainstem chemoreceptors control respiration even within the normal pH range in several species. The correlation between pH and ventilation in our patients confirms that acid-base homeostasis contributes to resting ventilation.